Plant communities of harsh and favorable environments: Peculiarities of the organization, dominant structure and its relation to species richness

It is believed that plant communities of harsh and favorable habitats are organized in different ways. The first - mainly by abiotic environment and random processes (S-model organization), the latter - mainly by competition (C-S-R-model). It can be expected that the characteristics of the organization have an impact on the structure of the dominance of plant communities and its relationship to species richness. The aim of our study was to test this assumption with the example of herbaceous plant communities of relatively stable habitats of several regions and high-altitude belts of the Western Caucasus (alpine snowbeds, heaths and meadows, sub alpine meadows and marshes, low-grass meadows, grassy tiers of forests, steppes). As a criterion of the degree of severity of the environment, the aboveground biomass of communities at the time of its maximum development was used. The relative participation of species was estimated as the ratio of their biomass to the total aboveground biomass on the site (D) or to the biomass of the community remaining after removal of the biomass of higher rank species (K) from it. Under the structure of dominance, relative participation in community of the first rank species (degree of dominance) was understood, as well as the ratio of the values of D and K for two or three species with the largest participation in aboveground community biomass. The following results were obtained: 1. If the dominance structure is estimated on the basis of the D, then it varies widely and is almost unrelated to the production of the habitats (environmental conditions). 2. In C-S-R-communities K values for species of first rank (K-1) are on average higher than for less significant species (K-2 and K-3), in S-communities they are on average approximately the same. Moreover, if we consider C-S-R- and S-communities separately, then (K-1 - K-2) values are not related to the aboveground biomass of communities. The assumption is made that (K-1 - K-2) value can be considered as index of the degree of expression in the dominants of the C-strategy features. 3. The spatial variability of the parameters of the dominance structure is higher in areas of harsh habitats than in favorable. 4. C-S-R-communities are characterized by a closer relationship between degree of dominance and species richness than S-communities. Our results suggest that this is due to both more intensive interspecies competition in C-S-R-communities and to the C-strategy of dominant species.